introduce progress callbacks on Deferreds (just as suggested in the Promise/A proposal). It's kind of a first step before being able to support xhr2 upload and is a much needed addition when dealing with asynchronous tasks that take a non-trivial amount of time to complete and would profit from progression notifications,

while working on adding progress callbacks I realized that, while different, the logic for the progress callbacks list wasn't that far away from the one of the other two callbacks lists (the only real difference being that the progress callbacks can be called several times, while done and fail ones are only fired once). This brought me to scrap $._Deferred and go for a much more generic $.Callbacks. The advantage is that I can now implement done, fail and progress callbacks with the same base object. It also gives us an opportunity to factor our different callbacks lists throughout jQuery (meaning more robustness and a diminished code size). It can also be made public so that users can start creating callbacks lists easily.

All my experiments are located in the 1.7/callbacks branch. Still a work in progress, of course. Progress callbacks are in (though buggy atm because I screwed the memory logic of repeatable Callbacks but I'm working on it), $.when supports progress callbacks already. All instances of $._Deferred have been replaced by $.Callbacks objects.

@jaubourg: jörn, dave, and I were wondering if all of the flags were necessary. If they are not going to be used in 1.7, we may want to wait until 1.8 to add them. Could you provide more details on their usefulness?

unique, stopOnFalse and addAfterFire are useful for event-like lists (unique ensures the same handler will only be called once, stopOnFalse ensures you can "interrupt" firing the list, addAfterFire makes it so when you add a new callback to the list while it is firing then said callback won't be fired as part of current run).

relocate I added after a discussion with Yehuda at jqcon when he told me he had a need for "Deferreds that would have some kind of callback re-ordering facility" (a reverse option would be nice to have callbacks called from the last to the first one in the same vein btw).

queue is more of an experimental flag I put hoping, at the time, that we may be able to replace existing queues with $.Callbacks-based queues.

Apart from queue, these flags are pretty generic in what they do. Just tried to cover as much ground as possible. Not sure about the flag names to be honest, as usual, any help with that is more than welcome.

I'm also not sure the complexity added with the filter parameter is really needed. Just let me know what your review brings and I'll gladly cut (this was the sense of my request for a code review).